Wizard/progressive stepper: Should states only be controlled by back button?

Suppose a stepper has 6 steps. Can the user directly jump to the 4th step or should he follow the same flow with a click on "save and next". Same flow for the reverse order as well.

For example:

Should step 2 or 3 be clickable by the user? Suppose the user is in step 3 and wants to make some changes in step 1. Should the user click the back button twice in order to reach step 1 or can user jump directly to any step by clicking on the number shown below?

What should be the ideal flow for a wizard?